Contributions of spin-lattice relaxation to the echo decay of planar Cu in high-temperature superconductors.

Measurement of T2G, the Gaussian component of the spin-echo envelope of planar Cu nuclei in high-temperature superconductors, gives important information about the real part of the Cu electron spin susceptibility. In the traditional picture of the planar Cu echo decay, the internuclear coupling is assumed to remain static with respect to spin-lattice relaxation and mutual exchange fluctuations. In some circumstances, however, this assumption breaks down. We calculate the internuclear corrections arising from spin-lattice relaxation to the conventional theory of T2G and show that T2G can be easily corrected for these effects. We argue that mutual exchanges due to the perpendicular indirect couplings are suppressed in these materials. For YBa2Cu4O8, we find a correction on the order of 10% in T2g and using the corrected values we find that the isotope ration 63T2G/65T2G agrees with theory.